Gas turbine blades, are exposed to high temperature combustion gases, and consequently are subject to high thermal stresses. Methods are known in the art for cooling the blades and reducing the thermal stresses. Typically high pressure air, discharged from a compressor, is introduced into an interior of an air-cooled blade from a blade root bottom portion. The high pressure air, after cooling a shank portion, a platform and an airfoil, flows out of fine holes provided at a blade face, or out of fine holes provided at a blade tip portion. Also, fine holes can be provided at a blade trailing edge portion of the blade, through which the high pressure air flows to cool the trailing edge of the blade. Fine holes can be provided on the platform surface for cooling. Thus, the high pressure air cools the metal temperature of the moving blade.
Highly cooled gas turbine blades experience high temperature mismatches at the interface of the hot airfoil and the relatively cooler shank portion of the platform. These high temperature differences produce thermal deformations at the platform, which are incompatible with those of the airfoil. In addition to thermal stresses large centrifugal forces act on the blade during operation adding to the stresses in the blade. When the airfoil is forced to follow the displacement of the shank and platform, high thermal stresses occur on the airfoil, particularly in the thin trailing edge region. These high thermal stresses are present during transient engine operation as well as steady state, full speed, full load conditions, and can lead to crack initiation and propagation. These cracks potentially can ultimately lead to catastrophic failure of the component.
The U.S. Pat. No. 5,947,687 discloses a gas turbine moving blade (FIGS. 1-3) having a groove on the trailing side of the platform of a turbine blade, designed to suppress a high thermal stress at the attachment point of the airfoil trailing edge and platform that occurs during transient operating conditions, i.e., starting and stopping of the turbine. This groove extends along the entire length of the platform, from the pressure side (typically with a concave curvature) of the blade to the suction side (typically with a convex curvature) along a circumference of the turbine, typically parallel to a plane of rotation of the turbine. In operation there is no effective seal between the trailing edge of the platform and subsequent vane platform or heat shield downstream of blade. The groove is typically open to a gap, which is purged by cooling air, and is facing the hot gas path of the turbine. If the purge flow is interrupted or the pressure distribution on the hot gas side is not as intended hot gas can be ingested through the gap and lead to local overheating of the groove and potentially overheating of the blade foot as well as of the turbine rotor.
Below the groove the turbine blade is connected to the rotor. The mechanical connection can for example be done with fir tree having a tapered form, with broached serrated edges providing multiple load-bearing faces. Below or between the feet of the blades cavities to supply pressurized cooling air to the blade are provided. To the axial downstream end of the blade these cavities can for example be closed by a shiplap, i.e. an overlap extending from one blade foot in circumferential direction beyond the neighboring blade foot. A shiplap makes assembly and disassembly of blades, especially of individual blades for repair difficult. In addition, a shiplap has limited sealing capabilities as the overlap has practically no mechanical flexibility.
A turbine heat shield as known for example from the EP1079070 is a device for separating a space region through which hot working medium flows from a preferably coolable space region inside a rotor arrangement of a gas turbine.
Such a heat shield arrangement has at least two rotor discs, which are arranged one behind the other in the axial direction, can be fixedly connected to one another by means of at least one connecting region and are spaced apart from one another at least in the region of their radial circumferential edges. A heat shield arrangement further is of sheet-like design, is arranged between two adjacent rotor discs and has two connecting edges, along which the heat shields can be brought into operative connection in each case in the region of the circumferential edges of the adjacent rotor discs, and which covers an intermediate space which extends on the rotor side between the two rotor discs. The heat shield arrangements serve to shape the hot-gas passage provided in the interior of a gas turbine at its diameter facing the rotor and protect structural parts of the rotor from overheating.
The known heat shield designs and turbines with such heat shields require purging of the axial downstream end of the blade foot below the platform. The purge air used has a detrimental effect on turbine power and efficiency. In addition any mechanical defect or change in the purge air supply can cause insufficient local purging resulting in a local overheating of the downstream end of the blade or of the rotor disk holding the blade.